1. Field of the Invention
The present invention relates to a developer unit which develops an electric latent image on the surface of a photoreceptor by using a developer containing a magnetic carrier and a non-magnetic toner and to a toner concentration controller used for the developer unit.
2. Description of the Prior Art
Typically, the toner concentration of a developer unit of this type is determined based on a result of a comparison between a predetermined reference value and a toner-to-carrier ratio in a mixture of toner and carrier detected by a permeability sensor. The output of the permeability sensor increases as the toner concentration decreases, and decreases as the toner concentration increases. Therefore, when the sensor output is higher than the reference value, toner is replenished to maintain the toner concentration constant. The toner replenishment is performed by controlling ON/OFF of a toner replenishing motor, that is, by repeating an ON period and an OFF period alternately within a predetermined period of time.
However, the toner consumption greatly differs according to whether one-side copying or both-side copying is performed. This applies to the paper size and the preset copy quantity. Therefore, in these cases, with the same amount of toner replenishment, images may be faint because of insufficient toner, or the fogging on images or the locking of the developer unit (a condition in which the rotary members in the developer unit have difficultly in rotating because of excessive toner) may occur because of excessive toner replenishment.